Have you tried:
1. deleting/renaming your .htaccess. it's possible that a .htaccess rule could be the source of the problem.
2. deactivating all plugins to see if this resolves the problem? If this works, re-activate the plugins one by one until you find the problematic plugin(s).
3. switching to the default theme to rule out any theme-specific problems? If you don't have access to your admin panel, access your server via FTP or SFTP, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the Default theme (Twenty Ten theme) to activate and hopefully rule-out a theme-specific issue
4. resetting the plugins folder by FTP or phpMyAdmin? Sometimes, an apparently inactive plugin can still cause problems.
5. re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress?